Written and edited by experts in the field, this collection from Cold Spring Harbor Perspectives in Biology examines the molecular and cellular details of the oscillatory machinery and how they control aspects of our physiology. Contributors discuss the cell-autonomous generation of circadian time and how those mechanisms interact with circuit-level mechanisms in the suprachiasmatic nucleus. Links between circadian and redox systems, as well as the relationship between circadian oscillations and pathogenic infections, are also covered.